372 



NATURE 



[November 17, 192 1 



is 51 mm. The length from the socket of the 

 median incisor to a line xlrawn across the back 

 of the third molars is also 51 mm. The whole 

 dentition is much affected wfth caries, and the 

 disease has spread to the tooth-sockets, which 

 are pierced in some places. 



The lower jaw is unfortunately absent, but the 

 size of the palate and the extent of the temporal 

 fossae show that it must have been massive. Even 

 the Heidelberg jaw is slightly narrower and 

 shorter than this must have been. 



Although the new skull from the Rhodesian 

 cave so much resembles that of Neanderthal man, 

 the shape of the brain-case and the position of the 

 foramen magnum are so different that we may 

 hesitate to refer the two skulls to the same race. 

 This hesitation seems to be justified when the 

 associated limb-bones are considered, for the tibia 

 is long and slender, of the typically modern type, 

 and the extremities of the femur do not differ in 

 any essential respect from the corresponding parts 

 of a tall and robust modern man. They are thus 



very different from the tibia and femur of Nean- 

 derthal man found in the caves of Belgium and 

 France. As the skull appears to postulate an erect 

 attitude, the congruous limb-bones may well be 

 referred to it. We therefore recognise in the 

 Rhodesian cave man a new form which may be 

 regarded as specifically distinct from Homo nean- 

 derthalensis, and may be appropriately named 

 Homo rhodesiensis. 



The precise systematic positfon of this new species 

 of primitive man can be determined only by further 

 discoveries. It has, however, been pointed out 

 by Prof. Elliot Smith that the refinement of the 

 face was probably the last step in the evolution 

 of the human frame. The newly discovered 

 Rhodesian man may therefore revive the idea that 

 Neanderthal man is truly an ancestor of Homo 

 sapiens; for Hom.o rhodesiensis retains an almost 

 Neanderthal face in association with a more 

 modern brain-case and an up-to-date skeleton. He 

 may prove to be the next grade after Neanderthal 

 in the ascending series. 



By 



Problems of Physics.^ 



Prof. O. W. Richardson, D.Sc, F.R.S. 



O ELATIVITY Is the revolutionary movement 

 An. in physics which has caught the public eye, 

 perhaps because it deals with familiar conceptions 

 in a manner which for the most part is found 

 pleasantly incomprehensible. But it is only one 

 of a number of revolutionary changes of com- 

 parable magnitude. Among these we have to 

 place the advent of the quantum. The various 

 consequences of the electronic structure of matter 

 are still unfolding themselves to us, and are 

 increasing our insight into the most varied phe- 

 nomena at a rate which must have appeared 

 incredible only a few decades ago. 



The enormous and far-reaching importance of 

 the discoveries being made at Cambridge by 

 Sir Ernest Rutherford cannot be over-emphasised. 

 These epoch-making discoveries relate to the 

 structure and properties of the nuclei of atoms. 

 At the present time we have, I think, to accept 

 it as a fact that the atoms consist of a positively 

 charged nucleus of minute size, surrounded at a 

 fairly respectful distance by the number of elec- 

 trons requisite to maintain the structure electri- 

 cally neutral. The nucleus contains all but about 

 one-two-thousandth part of the mass of the atom, 

 and Its electric charge Is numerically equal to 

 that of the negative electron multiplied by what 

 is called the atomic number of the atom, ' the 

 atomic number being the number which Is 

 obtained when the chemical elements are enume- 

 rated In the order of the atomic weights ; thus 

 hydrogen=i, helium = 2, Hthium = 3, and so on. 

 Consequently the number of external electrons in 

 the atom is also equal to the atomic number. The 

 evidence, derived from many distinct and dis- 

 similar lines of Inquiry, which makes It necessary 



1 Abridged from the presidential address delivered to Section A (Mathe- 

 matics and Physics) of the British Association at Edinburgh on September 9. 



to accept the foregoing statements as facts, will 

 be familiar to members of this Section of the 

 British Association, which has continually been 

 in the forefront of contemporary advances In 

 physical science. 



The diameters of the nuclei of the atoms are 

 comparable with one-millionth of one-millionth 

 part of a centimetre, and the problem of finding 

 what lies within the Interior of such a structure 

 seems at first sight almost hopeless. It is to this 

 problem which Rutherford has addressed himself 

 by the direct method of bombarding the nuclei of 

 the different atoms with the equally minute high- 

 velocity helium nuclei (alpha-particles) given off 

 by radioactive substances, and examining the 

 tracks of , any other particles which may be 

 generated as a result of the impact. A careful 

 and critical examination of the results shows that 

 hydrogen nuclei are thus expelled from the nuclei 

 of a number of atoms such as nitrogen and phos- 

 phorus. On the other hand, oxygen and carbon 

 do not eject hydrogen under these circumstances, 

 although there Is evidence in the case of oxygen 

 and nitrogen of the expulsion of other sub-nuclei 

 whose precise structure is a matter for further 

 inquiry. 



The artificial transmutation of the chemical 

 elements Is thus an established fact. The natural 

 transmutation has, of course, been familiar for 

 some years to students of radio-activity. The 

 philosopher's stone, one of the alleged chimeras 

 of the mediaeval alchemists, is thus within our 

 reach. But this is only part of the story. It 

 appears that in some cases the kinetic energy of ; 

 the ejected fragments Is greater than that of the 

 bombarding particles. This means that these 

 bombardments are able to release the energy 

 which is stored in the nuclei of atoms. Now, we 



NO. 2716, VOL. 108] 



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